Universal Spectral Correlations in Diffusive Quantum Systems

We have studied numerically several statistical properties of the spectra of disordered electronic systems under the influence of an Aharonov Bohm flux $\varphi$, which acts as a time--reversal symmetry breaking parameter. The distribution of curvatures of the single electron energy levels has a modified Lorentz form with different exponents in the GOE and GUE regime. It has Gaussian tails in the crossover regime. The typical curvature is found to vary as $-E_c\ln (E_c\varphi^2/\Delta)$ ($E_c$ is the Thouless energy and $\Delta$ the mean level spacing) and to diverge at zero flux. We show that the harmonics of the variation with $\varphi$ of single level quantities (current or curvature) are correlated, in contradiction with the perturbative result. The single level current correlation function is found to have a logarithmic behavior at low flux, in contrast to the pure symmetry cases. The distribution of single level currents is non--Gaussian in the GOE--GUE transition regime. We find a universal relation between $g_d$, the typical slope of the levels, and $g_c$, the width of the curvature distribution, as was proposed by Akkermans and Montambaux. We conjecture the validity of our results for any chaotic quantum system.